Recently, microreactor technology has received a great deal of attention. Most of the research carried out in this field is concerned with engineering aspects of microreactors. Microreactors consist of a series of small (10-1000μm) channels connected in various geometries that allow for the spatial and temporal manipulation of small amounts of fluids and reagents.[1] Extremely fast mixing by virtue of a short diffusion path is also an important advantageous feature of microreactors for highly homogeneous preparation.[2] We pay attention to the microreactor (MR), which is one of a solution synthesis method and has advantages that the proper pH value and temperature can be maintained during the reaction. Before, we proposed the combinatorial MR and reported that it enables the automatic and precise program control of reaction conditions such as temperature, pH and mixture ratio of sources [3]. In the reaction parameters, pH value is one of the key parameters, since it affects the reaction rate and the formation of crystal nucleus. Depending on the pH value, numbers of emission centers were changed, too. In other words, the solution pH control means number of emission centers control. In our former MR system, a pH value has been measured at the precursor product. A structure of mixer element, at where solution sources meets and the reaction takes place, is also important. In this study, to achieve more precise pH control, an in-situ pH monitoring of reacting solution has been attempted to use MR for YVO4: Bi, Eu nanophosphors preparation. A mixer glass cell has also been designed to realize the quick mixture of sources, which is MR system. In-situ pH monitor is not only the powerful tool for nanophosphors production process control but also every chemical production process. YVO4: Bi, Eu nanophosphor samples were synthesized by hybrid use of MR system and solvothermal method. The precursor was prepared by citric-acid-gel method using the MR system. The system consists of liquid sources, syringe pumps, a mixer cell, a mixing/reaction tube, and an auto-sampler. The mixer cell having a 1-mm-width Y-branch channel was fabricated by sandblast method with glass substrates. A pH sensor was placed at the outlet of the mixer cell. The acid solution (pH=5) sources and the basic solution (pH=12) sources were prepared Y(CH3COO)3°E4H2O, Eu(CH3COO)3°E4H2O, C6H5Na3O7°E2H2O, Bi(NO3)3°E5H2O, C2H6O2, H2O and Na3VO4, NaOH, H2O, respectively. These two sources were mixed at Y-branch and passed through a Teflon tube (1000μm diameter) in an ultrasonic bath maintained at 85 ºC. The in-situ pH monitor was performed during the synthesis. The monitored pH level was about 8.5. The complex ions were formed several types coordinate depend on pH levels in general. Due to the small reaction channels, the pH levels are precisely monitoring during the reaction, that is in-situ pH monitored in MR. The obtained precursor was carried out solvothermal process at 130 ºC for 6 h in the autoclave. After quenching to a room temperature, nanophosphor particles were collected by centrifugation. The spherical crystal grains of the YVO4: Bi, Eu nanophosphor sample were observed about 50 nm, which consists of smaller crystallites of about 10 nm in diameter. The observed XRD patterns of YVO4 are identified as those due to the tetragonal zircon structure of YVO4. According to the PL and PLE spectra, sharp emission peaks are found in the spectral range from 580 nm to 720 nm, and all peaks are attributed to the 4f6-4f6 electron transition of the Eu3+ centers. The broad PLE band is thought to be due to YVO4 host (250 - 300 nm) and charge transfer state of Bi3+ centers (300 - 400 nm) [4]. It is particularly worth noting of MR synthesis that the Bi3+center spectrum becomes higher than the previous reports. The internal quantum efficiency is 28 % at an excitation wavelength of 365 nm. It should be noted that the reproducibility of MR synthesis is entirely improved by employing the in-situ pH monitoring. REFERENCES [1] B. P. Mason, et al., Chem. Rev. 107, 2300, (2007) [2] J. Yoshida, et al., Chem. Eur. J. 14, 7450, (2008) [3] H. Okura, et al., Proc. IDW’10, 1097 (2010) [4] G. Blasse, A. Bril, J. Chem. Phys., 48, 217 (1968)
Over past few years, nano phosphors have been attracting attention as candidate materials for in vivo biological imaging, and several types of phosphor material have been investigated. Organic phosphor materials have an excellent luminescent efficiency, however their protein has a discoloration problem during the observation. Semiconductor quantum dot phosphors also have a problem in practice, that is, it contains harmful elements in a human body. Therefore, the development of highly stable and harmless phosphors are desired.[1]An ultraviolet (UV) light, which is usually used for exciting both organic fluorescent materials and semiconductor quantum dot phosphors, damages the human body. In addition, it is hard to observe deep part of a human body by using a UV light. To solve these problems, it is effective to use a high bio-permeability near-infrared region (so called biological window).[2] Therefore, the phosphors having the excitation and emission band in a near-infrared region must be developed. The particle size of 30 - 200 nm is also needed for drug delivery system. [3]We previously reported the synthesis of the nano phosphor materials synthesized by the microreaction (MR) method, which is one of the solution synthesis methods. [4] The MR method has an advantage to perform a homogeneous reaction and to precisely control the reaction conditions such as temperature and pH. A higher quantum efficiency has been obtained by MR method compared to the traditional solution synthesis. [5] In this study, we pay attention to YVO4:Nd,Yb as the vivo bioimaging nano phosphor. The pH value and temperature during the MR synthesis have been varied, and the relation between these reaction parameters and luminescent properties has been investigated.The precursor of YVO4:Nd,Yb was prepared by a citric-acid-gel method using the MR system. The MR system consists of liquid sources, syringe pumps, a mixer cell, a mixing/reaction tube, and an auto-sampler. The 1-mm-width T-branch channel was used as the mixer cell. The cell was prepared by a sandblasting method with glass substrates. A pH sensor was placed at the outlet of the mixer cell. Y(CH3COO)3·4H2O, Nd(CH3COO)3·H2O, Yb(CH3COO)3·4H2O and C6H5Na3O7·2H2O was used as the acid source (pH=5). Na3VO4and NaOH were used as the basic solution (pH=12) sources. These two sources were mixed at the T-branch mixer, and the mixture solution was passed through a Teflon tube (1000μm diameter) in an ultrasonic bath maintained at 85 ºC. The in-situ pH monitor was performed during the synthesis.The obtained precursor was poured into the autoclave. The autoclave treatment was carried out at 130 ºC for 6 h. The synthesized nano phosphor particles were collected by centrifugation and freeze-drying. The photoluminescent (PL) characteristics in the near-infrared region were measured using a spectrometer (JASCO FP-8700). Figure 1 shows the X-ray diffraction (XRD) pattern of the synthesized samples. Only diffraction peaks due to the tetragonal zircon structure of YVO4 are observed. The grain size (D50) of the sample is about 50 nm as shown in Fig. 2. From the scanning electron microscope (SEM) measurements, it was confirmed that the spherical polycrystal grain consists of smaller crystallites of about 10 nm. Figure 3 shows the typical PL result of the obtained YVO4:Nd, Yb nano phosphor sample. Three PL peaks at about 905, 1070, and 1350 nm are due to the 4F3/2→4I9/2, 4I11/2, and 4I13/2 electron transitions in Nd3+ centers, respectively. That at 985nm is due to the 2F5/2→2F7/2 transition in Yb3+ centers. The PL excitation spectrum (PLE) monitored at 1070 nm is shown in Fig. 4. The excitation bands peaking at about 435, 480, 530, 598, 685, 756 and 810 nm are attributed to the transitions of 4I9/2→4G11/2, 4G9/2, 4G7/2, 4G5/2,4F9/2, 4F7/2 and 4F5/2 in Nd3+ centers, respectively. The board excitation band located at 300-350 nm is due to the YVO4 host absorption. It was confirmed that the proper temperature and pH values are 50 ℃ and pH 8.5, respectively. It concludes that YVO4: Nd, Yb nano phosphor synthesized by MR method is suitable for in vivo bioimaging. References [1] T. Zako, et al., Biomateral. Sci., 3, p. 59 (2015).[2] A. M. Smith, et al., Nat. Nanotech., 4, p. 170 (2009).[3] M Gaumet, et al., Eur J PHARM BIOPHARM p.69 (2008).[4] H. Okura et al., Proc. IDW’10, p. 1097 (2010).[5] T. Ishigaki et al., ECS Abstract, p. 2156 (2014). Figure 1
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